Interferometer having multiple scan carriages

ABSTRACT

An interferometer includes a fixed assembly including a base, a beam splitter assembly and a fixed mirror, and a movable assembly including an upper scan carriage, a lower scan carriage and a movable mirror connected to the lower scan carriage. The pair of inner bearing flexures is connected to the base and the upper scan carriage, enabling movement of the upper scan carriage relative to the base, and the pair of outer bearing flexures is connected to the upper and lower scan carriages, enabling movement of the lower scan carriage relative to the upper scan carriage. The movement of the upper and lower scan carriages enable a scan movement of the movable mirror in a scan direction restricted such that the scan movement maintains a plane containing the movable mirror parallel to planes containing the movable mirror at respective distances between the movable and fixed assemblies during the scan movement.

BACKGROUND

Michelson interferometers are used in many commercial applications.Performance characteristics and stability limitations of various designsare well known and understood. Slight misalignments of optical elementsin a conventional interferometer cause modulation changes that maysignificantly affect the performance of the interferometer. There havebeen numerous attempts in the design of commercial interferometers,Michelson interferometers included, to reduce misalignments and/or theeffects of misalignments. Some of these attempts include passive means,such as using cube corner mirrors, retro-mirrors, and/or other means tocompensate for undesirable effects. Other attempts have used activemeans such as dynamic mirror alignment or active thermal control, amongothers. Alternatively, adjustment mechanisms are available to enableperiodic or necessary reestablishment of the relationships of theoptical elements to maintain acceptable alignment conditions.

Generally, Michelson interferometers produce an alternating opticalsignal by splitting an input beam of light into two portions, inducingan alternating path difference in one of the portions, and recombiningthe portions at the exact point of initial splitting. Maintainingflatness and consistent geometric relationship (to a wave front) of themirror element that produces the path difference during a scan (e.g.,movable mirror) is important to system performance, stability andultimately instrument data quality. Any short or long term change(commonly referred to as optical instability) in the geometricrelationship or flatness of either the fixed or movable mirrors to thewave front may produce compromised results. Similar results occur whenthe beam splitter changes flatness or angle relative to the wave front.

Spectral resolution of an interferometer is related to the distance themovable mirror moves during the scan. In the field of Fourier transforminfrared (FTIR) spectroscopy instrument design, in particular, movementof the movable mirror is typically achieved via a mechanical bearing.There are many bearing implementations having a wide spectrum of costsand complexity.

Flat springs (bearings or bearing flexures) have been used withinterferometers. U.S. Pat. No. 7,630,081, to Ressler et al. (Dec. 8,2009), which is hereby incorporated by reference, is an example ofconventional interferometers. U.S. Pat. No. 7,630,081 addresses use of apair of bearing flexures, including disclosure of material selection andgeometry, to achieve a high degree of performance and thermal/mechanicalstability at a relatively low cost and relatively small size. However,such conventional interferometers may have limited resolution capabilitydue to inherent bearing travel restrictions. Thus, there is a need forhigh performance, reliable interferometers that are capable of longerbearing travel providing greater resolution, e.g., for mid-levellaboratory markets.

SUMMARY

According to a representative embodiment, an interferometer includes afixed assembly and a movable assembly which is movable with respect tothe fixed assembly, a pair of inner bearing flexures and a pair of outerbearing flexures. The fixed assembly includes a base, a beam splitterassembly configured to split light emitted from a light source intofirst and second portions of light, and a fixed mirror for reflectingthe first portion of light. The movable assembly includes an upper scancarriage, a lower scan carriage and a movable mirror connected to thelower scan carriage for reflecting the second portion of light. The beamsplitter assembly is further configured to combine the reflected firstand second portions of light into a recombined radiation beam. The pairof inner bearing flexures has ends connected to the base and endsconnected to the upper scan carriage, enabling movement of the upperscan carriage relative to the base. The pair of outer bearing flexureshas ends connected to the upper scan carriage and ends connected to thelower scan carriage, enabling movement of the lower scan carriagerelative to the upper scan carriage. The movement of the upper scancarriage and the movement of the lower scan carriage enable a scanmovement of the movable mirror in a scan direction restricted such thatthe scan movement maintains a plane containing the movable mirrorparallel to planes containing the movable mirror at respective distancesbetween the movable assembly and the fixed assembly during the scanmovement. The beam splitter assembly, the fixed mirror and the movablemirror are positioned within a space between the planes containing thepair of inner bearing flexures while in respective flat conditions.

According to another representative embodiment, a Fourier transforminfrared spectroscopy system includes a light source configured to emitinfrared radiation, and an interferometer configured to receive theinfrared radiation and to provide recombined radiation comprising firstand second portions of reflected radiation having varying relativephases. The interferometer includes a beam splitter assembly, a fixedmirror, a movable mirror, upper and lower scan carriages, and adetection system for receiving the recombined radiation. The beamsplitter assembly is mounted to a stationary base and configured tosplit the infrared radiation emitted from the light source into firstand second portions of radiation and to recombine the first and secondportions of reflected radiation to provide the recombined radiation. Thefixed mirror is mounted to the stationary base and configured to reflectthe first portion of radiation to provide the first portion of reflectedradiation to the beam splitter. The movable mirror is configured toreflect the second portion of radiation to provide the second portion ofreflected radiation to the beam splitter, the movable mirror beingmovable with respect to the beam splitter in a scan direction to changethe phase of the second portion of reflected radiation relative to thefirst portion of reflected radiation. The upper scan carriage is mountedto the stationary base via a pair of inner bearing flexures and ismovable in a first arc motion with respect to the stationary base, thebeam splitter assembly, the fixed mirror and the movable mirror beingpositioned within a space between planes containing the pair of innerbearing flexures while in respective unflexed positions. The lower scancarriage is mounted to the upper scan carriage via a pair of outerbearing flexures and movable in a second arc motion with respect to theupper scan carriage, the second arc motion being substantially equal tothe first arc motion, while arcing in an opposite direction. The movablemirror is attached to the lower scan carriage such that movement of theupper scan carriage in the first arc motion and movement of the lowerscan carriage in the second arc motion enable movement of the movablemirror in the scan direction.

According to another representative embodiment, an interferometer of aFourier transform infrared spectroscopy system includes a fixedassembly, a movable assembly, a pair of inner bearing flexures, and apair of outer bearing flexures. The fixed assembly includes a base, abeam splitter configured to split light emitted from a light source intofirst and second portions of light, and a fixed mirror for reflectingthe first portion of light. The movable assembly, which is movable withrespect to the fixed assembly, includes an upper scan carriage, a lowerscan carriage and a movable mirror connected to the lower scan carriagefor reflecting the second portion of light. The beam splitter is furtherconfigured to combine the reflected first and second portions of lightinto a recombined radiation beam. The pair of inner bearing flexures hasupper ends connected to the base and lower ends connected to the upperscan carriage, enabling movement of the upper scan carriage relative tothe base. The pair of outer bearing flexures has upper ends connected tothe upper scan carriage and lower ends connected to the lower scancarriage, enabling movement of the lower scan carriage relative to theupper scan carriage. An upper point in an upper plane of the upper scancarriage defines an upper arc during the movement of the upper scancarriage, and a lower point in a lower plane of the lower scan carriagedefines a lower arc, equal and opposite to the upper arc, during themovement of the lower scan carriage, where the upper point and the lowerpoint are geometric conjugates, enabling a scan movement of the movablemirror such that a plane containing the movable mirror is maintainedparallel to a plurality of planes containing the movable mirror at acorresponding plurality of distances between the movable assembly andthe fixed assembly during the scan movement.

BRIEF DESCRIPTION OF THE DRAWINGS

The illustrative embodiments are best understood from the followingdetailed description when read with the accompanying drawing figures. Itis emphasized that the various features are not necessarily drawn toscale. In fact, the dimensions may be arbitrarily increased or decreasedfor clarity of discussion. Wherever applicable and practical, likereference numerals refer to like elements.

FIG. 1 illustrates a simplified cross-sectional side elevation view ofoptical elements in an interferometer, according to a representativeembodiment;

FIG. 2 illustrates a side isometric view of an interferometer, accordingto a representative embodiment;

FIG. 3 illustrates a side isometric view of the upper scan carriage andbase of an interferometer, according to a representative embodiment.

FIG. 4 illustrates a side isometric view of the lower scan carriage ofan interferometer, according to a representative embodiment.

FIG. 5 illustrates a front elevation view of a bearing flexure in aninterferometer, according to a representative embodiment;

FIG. 6 illustrates a side elevation view of an interferometer, accordingto a representative embodiment;

FIG. 7 illustrates a cross-section side elevation view of aninterferometer, according to a representative embodiment;

FIG. 8 illustrates an isometric cross-sectional view showing detail of afixed mirror and mechanism for moving it relative to a fixed assembly,according to a representative embodiment; and

FIG. 9 illustrates a top plan view of a movable beam splitter assemblyof an interferometer, according to a representative embodiment.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, illustrative embodiments disclosing specific details areset forth in order to provide a thorough understanding of embodimentsaccording to the present teachings. However, it will be apparent to onehaving had the benefit of the present disclosure that other embodimentsaccording to the present teachings that depart from the specific detailsdisclosed herein remain within the scope of the appended claims.Moreover, descriptions of well-known devices and methods may be omittedso as not to obscure the description of the example embodiments. Suchmethods and devices are within the scope of the present teachings.

Generally, it is understood that the drawings and the various elementsdepicted therein are not drawn to scale. Further, relative terms, suchas “above,” “below,” “top,” “bottom,” “upper,” “lower,” “left,” “right,”“vertical” and “horizontal,” are used to describe the various elements'relationships to one another, as illustrated in the accompanyingdrawings. It is understood that these relative terms are intended toencompass different orientations of the device and/or elements inaddition to the orientation depicted in the drawings. For example, ifthe device were inverted with respect to the view in the drawings, anelement described as “above” another element, for example, would now be“below” that element. Likewise, if the device were rotated 90 degreeswith respect to the view in the drawings, an element described as“vertical,” for example, would now be “horizontal.”

FIG. 1 illustrates a simplified cross-sectional side view of opticalelements in an interferometer, according to a representative embodiment.

Referring to FIG. 1, optical elements, including a beam splitter 4, amovable mirror 8, and a fixed mirror 9, in an interferometer 1 areillustrated according to a representative embodiment. Light 3 from alight source 2 is directed to the beam splitter 4, which includes a beamsplitting surface 5. The light 3 may be infrared radiation and the lightsource 2 may be a source of infrared radiation, for example. In anembodiment, the interferometer may be used for Fourier transforminfrared (FTIR) spectroscopy, for example. The light 3 is split by thebeam splitting surface 5 into a transmitted portion 7 a (first portion)and a reflected portion 6 a (second portion). The reflected portion 6 acontinues on to the movable mirror 8, which reflects the reflectedportion 6 a back onto itself as a reflected beam 6 b (reflected secondportion) that returns to the beam splitting surface 5. In a similarfashion, the transmitted portion 7 a continues on to the fixed mirror 9,which reflects the transmitted portion 7 a back onto itself as areflected beam 7 b (reflected first portion) that returns to beamsplitting surface 5. The reflected beams 6 b, 7 b are recombined at thebeam splitting surface 5 and at least a portion of the reflected beams 6b, 7 b is reflected as a recombined radiation beam 10. The recombinedradiation beam 10 then continues on to a sampling apparatus 11. Thesampling apparatus 11 modifies the recombined radiation beam 10 intosample encoded radiation 12 as a function of a sample 11 a in thesampling apparatus 11. The sample encoded radiation 12 continues on to adetection system 13. The sample 11 a is identified as a function of thesample encoded radiation 12. It is understood that changing the locationof the light source 2 with the sampling apparatus 11 and the detectionsystem 13 is known to those in the art.

When the respective optical distances from the beam splitting surface 5to the movable mirror 8 and the fixed mirror 9 are substantially equal,the recombined radiation beam 10 remains in phase, as there is nooptical retardation. When the movable mirror 8 moves either closer to orfurther from the beam splitting surface 5, while a planar surface of themovable mirror 8 maintains angular orientation to the reflected portion6 a, the reflection of the reflected beam 6 b returns to beam splittingsurface 5 and a retardation change is created. The retardation change inthe reflected beam 6 b modulates the recombined radiation beam 10.

When using the interferometer 1 as part of a rapid scanning FTIR system,as shown in FIG. 2, for example, the movable mirror 8 is typicallydriven at a constant velocity so as to modulate the recombined radiationbeam 10 in a known manner that may subsequently be Fourier transformedto recover frequency information of the recombined radiation beam 10and/or the sample encoded radiation 12. If one or more of the opticalelements including the beam splitter 4, the movable mirror 8, and/or thefixed mirror 9 change(s) position(s), or if the optical relationshipschange between the beam splitter 4, the movable mirror 8, and/or thefixed mirror 9, some amount of modulation change occurs in therecombined radiation beam 10. Such unintended changes in modulationproduce unwanted effects and, therefore, are undesirable.

FIG. 2 illustrates a side isometric view of an interferometer, accordingto a representative embodiment.

Referring to FIG. 2, interferometer 101, which may be used for Fouriertransform infrared spectroscopy, for example, includes fixed assembly140 and movable assembly 120, where the movable mirror 8 and a motorcoil assembly 125 are attached to the movable assembly 120. The fixedassembly 140 includes fixed housing or base 141 and a beam splittersupport 147, which is integrally combined with the base 141. The beamsplitter support 147 provides a guide for movable beam splitter assembly160 that includes selectable beam splitters 4 a, 4 b (not shown in FIG.2), each of which is configured to split the light 3 emitted from thelight source 2 into reflected portion 6 a and transmitted portion 7 a,as discussed above with reference to the beam splitter 4. The base 141further includes the fixed mirror 9 for reflecting the transmittedportion 7 a to provide the reflected beam 7 b. The movable assembly 120is movable with respect to the fixed assembly 140, and includes an upperscan carriage 130 and a lower scan carriage 150. In an embodiment, themovable mirror 8 is connected to the lower scan carriage 150 forreflecting the reflected portion 6 a to provide the reflected beam 6 b.The beam splitter assembly 160 is configured to combine the reflectedbeams 6 b, 7 b into recombined radiation beam 10 which is output to thesampling apparatus 11 and the detection system 13, as discussed above.

The interferometer 101 further includes a pair of inner hearing flexures131 a, 131 b (e.g., first inner bearing flexure 131 a and second innerbearing flexure 131 b) and a pair of outer bearing flexures 151 a, 151 b(e.g., first outer bearing flexure 151 a and second outer bearingflexure 151 b), Each of the inner bearing flexures 131 a, 131 b has alower end connected to the base 141 and an upper end connected to theupper scan carriage 130, enabling movement (oscillation) of the upperscan carriage 130 relative to the base with minimal scan friction in theplus/minus scan direction SD. Each of the outer bearing flexures 151 a,151 b has an upper end connected to the upper scan carriage 130 and alower end connected to the lower scan carriage 150, enabling movement(oscillation) of the lower scan carriage 150 relative to the upper scancarriage 130 with minimal scan friction in the plus/minus scan directionSD.

The inner bearing flexures 131 a, 131 b and the outer bearing flexures151 a, 151 b may be flat springs, for example, configured to flex orbend substantially to facilitate movement in a scan direction SD of themovable mirror 8. That is, the movement of the upper scan carriage 130and the movement of the lower scan carriage 150 enable a scan movementof the movable mirror 8 in the scan direction SD when an electromagneticforce is exerted on the motor coil assembly 125. The electromagneticforce is largely exerted either to the left or to the right, asillustrated by the scan direction SD, depending on the polarity of thevoltage exerted on the motor coil assembly 125, for example. The motorcoil assembly 125 may include a combination of a coil and magnetcomprising a linear drive motor operating under servo control thatimparts an alternating drive force to the lower scan carriage 150, forexample. In the depicted embodiment, as the lower scan carriage 150moves under motor drive force of the motor coil assembly 125, the outerbearing flexures 151 a, 151 b bend, thus imparting force into the innerbearing flexures 131 a, 131 b, which also bend. With each incrementalmovement of the lower scan carriage 150, this process continues untilthe requisite travel distance of the movable mirror 8 is achieved in onedirection. The polarity of the motor coil assembly 125 may then bereversed, causing a directional change until the requisite traveldistance of the movable mirror 8 in the opposite direction is achieved.

The scan movement is restricted such that it maintains a planecontaining a reflecting surface of the movable mirror 8 parallel toplanes containing the movable mirror 8 of the movable assembly 120 atall respective distances between the movable assembly 120 and the fixedassembly 140 during a scan operation. In other words, because of therestricted movement provided by the inner bearing flexures 131 a, 131 band the outer bearing flexures 151 a, 151 b, the optical relationshipbetween the beam splitter 4 a or 4 b and the movable mirror 8 remainssubstantially unchanged except for the optical retardation. Thus, thereflecting surface of the movable mirror 8 does not deflect angularly asthe movable assembly 120 is translated relative to the fixed assembly140. In an embodiment, the movable mirror 8, the fixed mirror 9 and thebeam splitter assembly 160 are positioned entirely in a space betweenplanes containing the pair of inner bearing flexures 131 a, 131 b whilein respective flat conditions (unflexed or neutral positions), as shown.

The depicted embodiment using two pairs of bearing flexures, e.g., innerbearing flexures 131 a, 131 b and outer bearing flexures 151 a, 151 b,may achieve longer movement distances appropriate for high resolutionspectroscopy for example, as compared to the shorter distancesappropriate for lower resolution spectroscopy of conventionalinterferometers using a pair of bearing flexures. The relationshipbetween resolution and requisite scan distance is L=1/[2(resolution)].Specifically, the distance a mechanical bearing must travel for 2wavenumber resolution is about ±0.25 cm, whereas for ½ wavenumberresolution the travel is about ±1 cm. In other words, this improvementin resolution requires a 4× increase in bearing travel distance, whichis achieved using the dual pairs of inner bearing flexures 131 a, 131 band outer bearing flexures 151 a, 151 b as in the depicted embodiment,while maintaining requisite scan parameters and interferometer designgoals.

FIGS. 3 and 4 illustrate side isometric views of the upper scan carriage130 (connected to the base 140) and the lower scan carriage 150,according to a representative embodiment.

The upper scan carriage 130 is shown attached to the inner bearingflexures 131 a, 131 b, which are also attached to the base 141 of thefixed assembly 140. The base 141 supports the beam splitter assembly160, discussed below. The upper scan carriage 130 defines an upperopening 137 through which the light 3 is able to pass to impinge on oneof the selectable beam splitters 4 a, 4 b. The inner bearing flexure 131a (also shown in FIG. 5) defines an opening 138 a through which themovable mirror 8 and at least a portion of the motor coil assembly 125may be inserted, and the opposing inner bearing flexure 131 b defines anopening 138 b which provides an unobstructed optical path of therecombined radiation beam 10 from the beam splitter assembly 160.

The lower scan carriage 150 is shown attached to the outer bearingflexures 151 a, 151 b. The movable mirror 8 and the motor coil assembly125 are attached to the lower scan carriage 150. The lower scan carriage150 defines a lower opening 159 in which the base 141 of the fixedassembly 140 may be positioned with sufficient clearance to enable thelower scan carriage 150 to move freely in relation to the base 141 whenthe upper ends of the outer bearing flexures 151 a, 151 b are attachedto the lower scan carriage 150, as shown in FIG. 2, discussed above. Theouter bearing flexure 151 a defines an opening 158 a providing clearancethrough which the movable mirror 8 and at least a portion of the motorcoil assembly 125 may be inserted, and the opposing outer bearingflexure 151 b defines an opening 158 b through which the recombinedradiation beam 10 may be transmitted unrestricted. The openings 138 a,138 b may be substantially aligned with the openings 151 a, 151 b,respectively.

FIG. 5 illustrates a front elevation view of an example of a bearingflexure in an interferometer, according to a representative embodiment.The bearing flexure shown in FIG. 5 is identified as the inner bearingflexure 131 a, although it is understood that the bearing flexure isrepresentative of any of the inner bearing flexures 131 a, 131 b and/orthe outer bearing flexures 158 a, 158 b. Generally, the inner bearingflexures 131 a, 131 b are substantially the same size and shape as oneanother, and the outer bearing flexures 151 a, 151 b are likewisesubstantially the same size and shape as one another. Further, the pairof inner bearing flexures 131 a, 131 b may be substantially the samesize and shape as the pair of outer bearing flexures 151 a, 151 b, asshown.

The representative inner bearing flexure 131 a includes the opening 138a, which may be substantially rectangular in shape. The opening 138 aprovides appropriate modulus characteristics for both bearing the weightof the movable assembly 120 and providing resistance to undesirableshocks, torques, and shears imposed by environmental forces. Althoughthe opening 138 a is rectangular in the illustrated embodiment, it isunderstood that other embodiments may include other shapes for theopening 138 a of the inner bearing flexure 131 a (as well as theopenings 138 b, 158 a, 158 b), such as oblong, elliptical, circular, orany other shape that might improve robustness and/or stability, withoutdeparting from the scope of the present teachings.

FIG. 6 illustrates a side elevation view of an interferometer, in anassembled state, according to a representative embodiment.

Referring to FIG. 6, the interferometer 101 includes the fixed assembly140 and the movable assembly 120, where the movable mirror 8 and themotor coil assembly 125 are attached to the movable assembly 120. Thefixed assembly 140 includes the base 141 and the beam splitter support147. The beam splitter support 147 includes upper flange 147 a and lowerflange 147 b confined to secure the movable beam splitter assembly 160,and guide movement of the beam splitter assembly 160, as discussedbelow. The movable assembly 120 is movable with respect to the fixedassembly 140, and includes the upper scan carriage 130 and the lowerscan carriage 150. The inner bearing flexures 131 a, 131 b are connectedto the upper scan carriage 130 and the base 141, and the outer bearingflexures 151 a, 151 b are connected to the upper scan carriage 130 andthe lower scan carriage 150. In an embodiment, the movable mirror 8 isconnected to the lower scan carriage 150 for reflecting the reflectedportion 6 a to provide the reflected beam 6 b.

As discussed above, the movable mirror 8 moves in the scan direction SDthrough operation of the motor coil assembly 125. The upper scancarriage 130 and the lower scan carriage 150 move together in a mannerthat maintains the plane of the movable mirror 8 parallel to all planescontaining the movable mirror 8 along the scan path moving in the scandirection SD (left or right, in the depicted orientation). Moreparticularly, the upper scan carriage 130 includes an upper plane 136and the lower scan carriage 150 includes a lower plane 156, where theupper plane 136 and the lower plane 156 are substantially parallel toone another when the inner bearing flexures 131 a, 131 b and the outerbearing flexures 151 a, 151 b are in the neutral or unflexed positions.The upper plane 136 and the lower plane 156 are also substantiallyparallel to the scan direction SD when the inner bearing flexures 131 a,131 b and the outer bearing flexures 151 a, 151 b are in the neutral orunflexed positions. Thus, when the movable mirror 8 moves (left orright) in the scan direction SD, an upper point UP in the upper plane136 of the upper scan carriage 130 defines an upper arc 138 in the samegeneral direction (left or right) as the movable mirror 8, while a lowerpoint LP in the lower plane 156 of the lower scan carriage 150 defines alower arc 158 in the same general direction (left or right) as themovable mirror 8. In an embodiment, the lower arc 158 is substantiallyequal and opposite to the upper arc 138 when the lower point LP and theupper point UP are geometric conjugates of each other. The upper pointUP follows the upper arc 138 and the lower point follows the lower arc158 in response to the simultaneous flexing or bending of the innerbearing flexures 131 a, 131 b and the outer bearing flexures 151 a, 151b.

In addition, the inner bearing flexures 131 a, 131 b define inner planes171 a, 171 b, respectively, in the neutral or unflexed position, and theouter bearing flexures 151 a, 151 b define outer planes 172 a, 172 b,respectively, in the neutral or unflexed position. In an embodiment, theinner planes 171 a, 171 b and the outer planes 172 a, 172 b aresubstantially parallel to one another. Further, the upper scan carriage130 contains a front plane 137 that is substantially parallel to theinner planes 171 a, 171 b of the inner bearing flexures 131 a, 131 b inthe neutral or unflexed position. The front plane 137 is alsosubstantially parallel to a front plane 142 of the base 141 when theinner bearing flexures 131 a, 131 b are in the neutral or unflexedposition. Similarly, the lower scan carriage 150 contains a front plane157 that is substantially parallel to outer planes 172 a, 172 b of theouter bearing flexures 151 a, 151 b in the neutral or unflexed position.The front plane 157 is also substantially parallel to the front plane137 when the inner bearing flexures 131 a, 131 b and the outer bearingflexures 151 a, 151 b are in the neutral or unflexed positions. In anembodiment, the front plane 137 of the upper scan carriage 130 remainsparallel to the front plane 142 of the base 141 throughout the scanmovement, and the front plane 157 of the lower scan carriage 150 remainsparallel to the front plane 137 of the upper scan carriage 130 (as wellas the front plane 142 of the base 141) throughout the scan movement.

FIG. 7 illustrates a cross-section side view of an interferometer,according to a representative embodiment.

Referring to FIG. 7, the interferometer 101 includes the fixed assembly140 and the movable assembly 120. The fixed assembly 140 includes thefixed mirror 9 and the beam splitter assembly 160, as well as variousaffixing means. In the depicted illustrative configuration, the affixingmeans include inner clamp members 132 a, 132 b and correspondingfasteners 133 a, 133 b (e.g., screws) for attaching lower ends of theinner bearing flexures 131 a, 131 b to the base 141. Multiple threadedholes 141 a, 141 b may be provided in the base 141 for affixing theinterferometer 101 to a frame, instrument housing, or base plate (notshown), for example.

The beam splitter assembly 160 includes a beam splitter 4 a and a beamsplitter 4 b (not shown in FIG. 7) on a beam splitter carriage 161slidably mounted to the beam splitter support 147 of the fixed assembly140 to enable selection of one of multiple selectable beam splitters 4 aand 4 b by sliding one of the beam splitters 4 a, 4 b into a path of thelight 3 emitted from the light source 2, as discussed below withreference to FIGS. 7 and 9. The beam splitter 4 a is mounted to asurface of the beam splitter carriage 161 by beam splitter fasteners 144a, 144 b and corresponding O-rings 145 a, 145 b. Opening 148 in the beamsplitter support 147 provides clearance for the recombined radiationbeam 10 output to the sampling apparatus 11 and the detection system 13,and opening 149 in the beam splitter support 147 and the base 141provides clearance for the transmitted portion 7 a transmitted to thefixed mirror 9 and the reflected beam 7 b reflected from the fixedmirror 9. The light 3 from the light source 2 enters the fixed assembly140 of the interferometer 101 via the upper opening 137, defined by theupper scan carriage 130, to reach the beam splitter 4 a.

The movable assembly 120 includes the upper scan carriage 130, the lowerscan carriage 150, the movable mirror 8, and the motor coil assembly125, as well as various affixing means. In an embodiment, the movablemirror 8 and the motor coil assembly 125 are connected to the lower scancarriage 150, and the movable mirror 8 is configured to reflect thereflected portion 6 a back onto itself as the reflected beam 6 b.Alternatively, the movable mirror 8 and the motor coil assembly 125 maybe connected to the upper scan carriage 130, and the lower scan carriage150 connected to the base 140, without departing from the scope of thepresent teachings.

In the depicted illustrative configuration, the affixing means of themovable assembly 120 include outer clamp members 152 a, 152 b andcorresponding fasteners 153 a, 153 b (e.g., screws) for attaching lowerends of the outer bearing flexures 151 a, 151 b to the lower scancarriage 150. The affixing means of the movable assembly 120 furtherinclude inner clamp members 139 a, 139 b, outer clamp members 154 a, 154b, and corresponding fasteners 155 a, 155 b (e.g., screws) for attachingupper ends of the inner bearing flexures 131 a, 131 b and upper ends ofthe outer bearing flexures 151 a, 151 b to the upper scan carriage 130,respectively. All components of the movable assembly 120 are rigidlyfastened to form a unit that moves together when actuated by anelectromagnetic force exerted on the motor coil assembly 125 in the scandirection SD, as discussed above. The use of motor coils for drivingmovable mirrors of interferometers to achieve retardation (e.g.,distance) between the fixed and movable assemblies 140, 120,respectively, is known.

Generally, the inner clamp members 132 a, 132 b and the fasteners 133 a,133 b act as means to affix the inner bearing flexures 131 a, 131 b tothe base 141. Movement between the movable and fixed assemblies 120, 140occurs as the inner bearing flexures 131 a, 131 b and the outer bearingflexures 151 a, 151 b deflect with relative motion of the upper scancarriage 130. Likewise, the outer clamp members 152 a, 152 b andfasteners 153 a, 153 b act as means to affix the outer bearing flexures151 a, 151 b to the lower scan carriage 150, and the inner clamp members139 a, 139 b, the outer clamp members 154 a, 154 b, and the fasteners155 a, 155 b act as means to affix the inner bearing flexures 131 a, 131b and the outer bearing flexures 151 a, 151 b to the upper scan carriage130, while allowing movement between the movable and fixed assemblies120, 140. In alternative embodiments, adhesives, braise solder, welding,epoxy, and extruded metal may be used to affix the inner/outer bearingflexures to the movable and fixed assemblies 120, 140.

The optical relationship between the beam splitter 4 a, 4 b and themovable mirror 8 may be satisfied when an angle of a surface of the beamsplitter 4 a, 4 b is maintained relative to an angle of a surface of themovable mirror 8. Similarly, the optical relationship between the beamsplitter 4 a, 4 b and the movable mirror 8 may be satisfied when anangle of an axis of a beam, from the beam splitter 4 a, 4 b toward themovable mirror 8 is maintained at a predetermined angle relative to thesurface of the movable mirror 8.

As discussed above, the beam splitter assembly 160, the fixed mirror 9,and the movable mirror 8 are positioned in the space between innerplanes 171 a, 171 b containing the inner bearing flexures 131 a, 131 b,respectively, in their flat condition.

Referring again to FIG. 5, in the illustrated embodiment, the clampedareas 32 of the representative inner bearing flexure 131 a further alignwith the edges of the clamp members 132 a, 139 a to define unrestrictedclamping length L. Clearance holes 33 are provided for the fasteners 133a, 155 a, and alignment holes 34 are provided to accommodate hightolerance pins (not shown) for precise positioning. Similarly, theclamped areas 32 and clearance holes 33 of the inner bearing flexure 131b would align with the edges of the clamp members 132 b, 139 b, andreceive the fasteners 133 b, 155 b. Also, the clamped areas 32 andclearance holes 33 of the outer bearing flexures 151 a, 151 b wouldalign with the edges of the clamp members 152 a, 152 b, 154 a, 154 b,and receive the fasteners 153 a, 153 b, 155 a, 155 b, respectively.

FIG. 8 illustrates an isometric cross-sectional view showing detail of afixed mirror and mechanism for moving it relative to a fixed assembly,according to a representative embodiment.

Referring to FIG. 8, the fixed mirror 9 is positioned in a fixed mirrorcavity 50. More specifically, a spherical surface portion 51 of the base141 contacts a spherical surface portion 52 of the fixed mirror 9. Six(6) adjustment screws, one of which is illustrated as 54, contact aconical surface 53 of the fixed mirror 9. In an embodiment, the screws54 are placed approximately symmetrically around a circumference of theconical surface 53. In this manner, the screws 54 may be used to adjustthe orientation of the fixed mirror 9. The fixed mirror adjustment maybe operated with servo or stepper motors (not shown), for example,although other means of adjusting the fixed mirror 9 may be incorporatedwithout departing from the scope of the present teachings.

FIG. 9 illustrates a top plan view of a movable beam splitter assemblyof an interferometer, according to a representative embodiment.

Referring to FIG. 9, the beam splitter assembly 160 includes the beamsplitter carriage 161, which is slidably mounted to the beam splittersupport 147 and/or the stationary base 141. Multiple beam splitters 4 a,4 b are positioned adjacent to one another on an upper surface of thebeam splitter carriage 161. The beam splitter carriage 161 is configuredto slide within the flanges 147 a, 147 b of the beam splitter support147 in a beam splitter positioning direction BSP, which is substantiallyperpendicular to the scan direction SD. Thus, the sliding operation ofthe beam splitter carriage 161 selectively places one of the beamsplitters 4 a, 4 b into the path of the light 3 emitted from the lightsource 2 (not shown in FIG. 9). The beam splitter carriage 161 may beoperated using a servo or stepper motor (not shown), for example,although other means of sliding the beam splitter carriage 161 may beincorporated without departing from the scope of the present teachings.

In the depicted arrangement, the beam splitter carriage 161 has beenpositioned to select the beam splitter 4 a. The beam splitters 4 a, 4 bmay be formed of different materials to transmit different frequenciesof interest of the light 3 emitted from the light source 2,respectively. For example, the beam splitters 4 a, 4 b may be configuredwith various combinations of ZnSe, KBr, or CaF2. Although FIG. 9 depictsthe two beam splitters 4 a, 4 b, it is understood that the beam splittercarriage 161 may include additional beam splitters without departingfrom the scope of the present teachings. It is further understood thatvarious embodiments of the interferometer 101 may be implemented using asingle, fixed beam splitter as opposed to selectable beam splitters.

During use, the two primary sources of optical misalignment andinstability are strains from mechanical and thermal stresses on theinterferometer 101. In an embodiment, the inner bearing flexures 131 a,131 b and the outer bearing flexures 151 a, 151 b are manufactured inmatched sets or pairs, and assembled to assure substantially preciselyrepeatable trajectories and relationships between the movable mirror 8on the movable assembly 120 and the beam splitter 4 a, 4 b and/or fixedmirror 9 on the fixed assembly 140, which remain precisely fixedrelative to each other.

Using the pair of inner bearing flexures 131 a, 131 b and the pair ofouter bearing flexures 151 a, 151 b, where respective inner planes 171a, 171 b bound a space that contains the interferometer optical elements(e.g., the beam splitter 4 a, 4 b, the fixed mirror 9, and the movablemirror 8) provides significant symmetry, minimum optical path lengthsfor the radiation, and minimum structural lengths. These features, alongwith proper selection of materials, minimize the effects of thermalchanges in the surrounding spaces. The illustrated embodiment furtherprovides a clear optical path for input or output radiation and amechanical means for conveniently driving the movable assembly 120including the movable mirror 8. Simultaneously, the symmetry andcompactness of the inner bearing flexures 131 a, 131 b and the outerbearing flexures 151 a, 151 b minimize strains from thermal andmechanical stresses. Significantly reduced strains result insignificantly improved optical stability.

Temperatures change constantly and are typically unpredictable in manyinstrument operating environments. Interferometers that need to functionin such environments typically need to be isolated from the temperaturechanges, have compensation means to counteract the effects of thechanges, and/or minimize the effects of such temperature changes.Historically, interferometers have been designed to be isolated fromenvironmental changes and to counteract the effects of environmentalchanges. The various embodiments herein help reduce and/or minimize theeffects of environmental changes.

In general, dimensions of a substance increase as the temperature of thesubstance itself increases. This relationship is typically statedaccording to the following formula: L=L_(o)(1+A(t−t_(o))), where L_(o)is the length of an object at temperature t_(o), A is a coefficient oflinear expansion, and L is the length of the object at temperature t.The coefficient of linear expansion A is known for most commonmaterials. In an infrared interferometer, the selection of the beamsplitter material determines the useful frequency range of theinstrument.

One commonly used material, Zinc Selenide (ZnSe) has A=7.2×10⁻⁶/C. Whenusing a typical one inch diameter (25.4 millimeters) beam splitter,there is approximately 1.5 microns change in diameter for each tendegree change on the centigrade scale if the beam splitter diameter isunrestricted. While this change appears relatively small, it is capableof creating enormous stresses if the beam splitter were constrained. Forexample, for a ZnSe beam splitter that is securely affixed to analuminum housing (a common practice for commercial FTIRs), the aluminumwould attempt to change at approximately three times the rate of theZnSe. The resulting stress, if not properly dissipated, could causesurface distortion and/or angular (e.g., tilt) change that may result inmodulation change and instability.

In an embodiment, the base 141, the upper scan carriage 130, the lowerscan carriage 150, the inner bearing flexures 131 a, 131 b, the outerbearing flexures 151 a, 151 b, the clamp members 132 a, 132 b, 139 a,139 b, 152 a, 152 b, 154 a, 154 b, the fasteners 133 a, 133 b, 153 a,153 b, 155 a, 155 b, the movable mirror 8, and the fixed mirror 9 eachmay be formed of steel, for example. In addition, it is contemplatedthat the movable mirror 8 and the fixed mirror 9 include a metalizedfilm for improved reflectivity in the infrared range, for example.

Steel and titanium have coefficients of thermal expansion much closer tothat of ZnSe. Therefore, using steel or titanium in place of aluminumhousing would reduce the strain differential between the ZnSe and thehousing. However, the benefit of improving strain differential bychanging from aluminum to steel or titanium to overcome other factorssuch as shape, thermal conductivity, thermal absorption, and thermalemissivity, among others, has not been previously demonstrated.Presumably, the improved strain differential has alone not achievedimproved results because other factors have been a source of opticalinstability. In that regard, the various embodiments described hereinsuggest that the roles of shape and symmetry are of equal, if notgreater, importance than the role of differential coefficients ofthermal expansion.

While the disclosure references exemplary embodiments, it will beapparent to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present teachings. Therefore, it should be understood that the aboveembodiments are not limiting, but illustrative.

What is claimed:
 1. An interferometer, comprising: a fixed assemblycomprising a base, a beam splitter assembly configured to split lightemitted from a light source into first and second portions of light, anda fixed mirror for reflecting the first portion of light; a movableassembly, movable with respect to the fixed assembly, comprising anupper scan carriage, a lower scan carriage and a movable mirrorconnected to the lower scan carriage for reflecting the second portionof light, wherein the beam splitter assembly is further configured tocombine the reflected first and second portions of light into arecombined radiation beam; a pair of inner bearing flexures having endsconnected to the base and ends connected to the upper scan carriage,enabling movement of the upper scan carriage relative to the base; and apair of outer bearing flexures having ends connected to the upper scancarriage and ends connected to the lower scan carriage, enablingmovement of the lower scan carriage relative to the upper scan carriage,wherein the movement of the upper scan carriage and the movement of thelower scan carriage enable a scan movement of the movable mirror in ascan direction restricted such that the scan movement maintains a planecontaining the movable mirror parallel to a plurality of planescontaining the movable mirror at respective distances between themovable assembly and the fixed assembly during the scan movement, andwherein the beam splitter assembly, the fixed mirror and the movablemirror are positioned within a space between the planes containing thepair of inner bearing flexures while in respective flat conditions. 2.The interferometer of claim 1, wherein an upper point in an upper planeof the upper scan carriage defines an upper arc during the movement ofthe upper scan carriage, and a lower point in a lower plane of the lowerscan carriage defines a lower arc, equal and opposite to the upper arc,during the movement of the lower scan carriage, wherein the upper pointand the lower point are geometric conjugates.
 3. The interferometer ofclaim 1, wherein the upper scan carriage contains a front plane that issubstantially parallel to a plane of the pair of inner bearing flexuresin an unflexed position, the front plane of the upper scan carriageremaining parallel to a front plane of the base throughout die scanmovement.
 4. The interferometer of claim 3, wherein the lower scancarriage contains a front plane that is substantially parallel to aplane of the pair of outer bearing flexures in an unflexed position, thefront plane of the lower scan carriage remaining parallel to the frontplane of the upper scan carriage throughout the scan movement.
 5. Theinterferometer of claim 1, wherein the first portion of light is atransmitted portion, and the second portion of light is a reflectedportion.
 6. The interferometer of claim 1, wherein the movable assemblyfurther comprises a motor coil assembly for enabling the scan movementof the movable mirror.
 7. The interferometer of claim 6, wherein each ofa first inner bearing flexure of the pair of inner bearing flexures anda first outer bearing flexure of the pair of outer bearing flexuresdefines a first opening for providing a clearance for at least one ofthe movable mirror and the motor coil assembly.
 8. The interferometer ofclaim 7, wherein a second inner bearing flexure of the pair of innerbearing flexures and a second outer bearing flexure of the pair of outerbearing flexures define a second opening for providing an unobstructedoptical path of the recombined radiation beam from the beam splitterassembly.
 9. The interferometer of claim 8, wherein a surface of themovable mirror does not deflect angularly as the movable assembly istranslated relative to the fixed assembly.
 10. The interferometer ofclaim 1, wherein the beam splitter assembly comprises a plurality ofselectable beam splitters.
 11. The interferometer of claim 10, whereinthe plurality of selectable beam splitters comprise different materialsto transmit different frequencies of interest of the light emitted fromthe light source, respectively.
 12. The interferometer of claim 10,wherein the beam splitter assembly further comprises a beam splittercarriage containing the plurality of selectable beam splitters, the beamsplitter carriage being slidably mounted to the fixed assembly to enableselection of one of the plurality of selectable beam splitters bysliding the one of the plurality of selectable beam splitters into apath of the light emitted from the light source.
 13. A Fourier transforminfrared spectroscopy system, comprising: a light source configured toemit infrared radiation; and an interferometer configured to receive theinfrared radiation and to provide recombined radiation comprising firstand second portions of reflected radiation having varying relativephases, the interferometer comprising: a beam splitter assembly mountedto a stationary base and configured to split the infrared radiationemitted from the light source into first and second portions ofradiation and to recombine the first and second portions of reflectedradiation to provide the recombined radiation; a fixed mirror mounted tothe stationary base and configured to reflect the first portion ofradiation to provide the first portion of reflected radiation to thebeam splitter; a movable mirror configured to reflect the second portionof radiation to provide the second portion of reflected radiation to thebeam splitter, the movable mirror being movable with respect to the beamsplitter in a scan direction to change the phase of the second portionof reflected radiation relative to the first portion of reflectedradiation; an upper scan carriage mounted to the stationary base via apair of inner bearing flexures and movable in a first arc motion withrespect to the stationary base, the beam splitter assembly, the fixedmirror and the movable mirror being positioned within a space betweenplanes containing the pair of inner bearing flexures while in respectiveunflexed positions; and a lower scan carriage mounted to the upper scancarriage via a pair of outer bearing flexures and movable in a secondarc motion with respect to the upper scan carriage, the second arcmotion being substantially equal to the first arc motion, while arcingin an opposite direction, wherein the movable mirror is attached to thelower scan carriage such that movement of the upper scan carriage in thefirst arc motion and movement of the lower scan carriage in the secondarc motion enable movement of the movable mirror in the scan direction;and a detection system for receiving the recombined radiation.
 14. TheFourier transform infrared spectroscopy system of claim 13, wherein eachinner bearing flexure of the pair of inner bearing flexures comprises anend connected to the stationary base and an end connected to the upperscan carriage, and wherein each outer bearing flexure of the pair ofouter bearing flexures comprises an end connected to the upper scancarriage and an end connected to the lower scan carriage.
 15. TheFourier transform infrared spectroscopy system of claim 14, wherein thepair of inner bearing flexures is positioned within the pair of outerbearing flexures, and the fixed mirror and the movable mirror arepositioned within a space between the inner bearing flexures when theinner bearing flexures are in an unflexed position.
 16. The Fouriertransform infrared spectroscopy system of claim 13, wherein the beamsplitter assembly comprises: a beam splitter carriage slidably mountedto the stationary base; and a plurality of beam splitters positioned inthe beam splitter carriage, wherein operation of the beam splittercarriage selectively places one of the plurality of beam splitters intothe path of the infrared radiation emitted from the light source. 17.The Fourier transform infrared spectroscopy system of claim 16, whereinthe plurality of selectable beam splitters comprise different materialsto transmit different frequencies of interest of the infrared radiationemitted from the light source, respectively.
 18. The Fourier transforminfrared spectroscopy system of claim 13, wherein the upper scancarriage contains a front plane that is substantially parallel to aplane of the pair of inner bearing flexures while in respective unflexedpositions and a front plane of the stationary base, the front plane ofupper scan carriage remaining parallel to the front plane of stationarybase throughout the movement of the upper scan carriage in the first arcmotion, and wherein the lower scan carriage contains a front plane thatis substantially parallel to a plane of the pair of outer bearingflexures while in respective unflexed positions and the front plane ofthe upper scan carriage, the front plane of lower scan carriageremaining parallel to the front plane of the upper scan carriagethroughout the movement of the lower scan carriage in the second arcmotion.
 19. An interferometer of a Fourier transform infraredspectroscopy system, the interferometer comprising: a fixed assemblycomprising a base, a beam splitter configured to split light emittedfrom a light source into first and second portions of light, and a fixedmirror for reflecting the first portion of light, a movable assembly,movable with respect to the fixed assembly, comprising an upper scancarriage, a lower scan carriage and a movable mirror connected to thelower scan carriage for reflecting the second portion of light, whereinthe beam splitter is further configured to combine the reflected firstand second portions of light into a recombined radiation beam; a pair ofinner bearing flexures having upper ends connected to the base and lowerends connected to the upper scan carriage, enabling movement of theupper scan carriage relative to the base; and a pair of outer bearingflexures having upper ends connected to the upper scan carriage andlower ends connected to the lower scan carriage, enabling movement ofthe lower scan carriage relative to the upper scan carriage, wherein anupper point in an upper plane of the upper scan carriage defines anupper arc during the movement of the upper scan carriage, and a lowerpoint in a lower plane of the lower scan carriage defines a lower arc,equal and opposite to the upper arc, during the movement of the lowerscan carriage, wherein the upper point and the lower point are geometricconjugates, enabling a scan movement of the movable mirror such that aplane containing the movable mirror is maintained parallel to aplurality of planes containing the movable mirror at a correspondingplurality of distances between the movable assembly and the fixedassembly during the scan movement.
 20. The interferometer of claim 19,wherein the upper scan carriage contains a front plane that issubstantially parallel to a plane of the pair of inner bearing flexuresin a neutral position, the front plane of the upper scan carriageremaining parallel to a front plane of the base throughout the scanmovement, and wherein the lower scan carriage contains a front planethat is substantially parallel to a plane of the pair of outer bearingflexures in a neutral position, the front plane of the lower scancarriage remaining parallel to the front plane of the upper scancarriage throughout the scan movement.